Method of fabricating electrically erasable programmable non-volatile memory cell structure

ABSTRACT

A NVM cell structure includes a semiconductor substrate having a first conductivity type, a first well region having a second conductivity type, a floating gate transistor and an erase gate region. The first well region is disposed on a first OD region of the semiconductor substrate. The erase gate region disposed on a second OD region of the semiconductor substrate includes a first doped region and at least one second doped region having the second conductivity type. The first doped region is disposed in semiconductor substrate and covers the second OD region, and the second doped region is disposed in the first doped region. The first doped region encompasses the second doped region, and a doping concentration of the second doped region is larger than a doping concentration of the first doped region.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/989,801, filed Jan. 7, 2016, which application claims thebenefit of U.S. provisional application No. 62/100,488 filed Jan. 7,2015, both of which applications are hereby incorporated herein byreference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a non-volatile memory (NVM) cellstructure, a NVM array structure and a method of fabricating a NVM cellstructure, and more particularly, to an electrically erasableprogrammable (EEP) NVM cell structure and an EEP NVM array structure anda method of fabricating the EEP NVM cell structure.

2. Description of the Prior Art

Non-volatile memory (NVM) is a type of memory that retains informationit stores even when no power is supplied to memory blocks thereof. Someexamples include magnetic devices, optical discs, flash memory, andother semiconductor-based memory topologies.

For example, U.S. Pat. No. 6,678,190 discloses a single-poly NVM havingtwo serially connected PMOS transistors wherein the control gate isomitted in the structure for layout as the bias is not necessary toapply to the floating gate during the programming mode. A first PMOStransistor acts as a select transistor. A second PMOS transistor isconnected to the first PMOS transistor. A gate of the second PMOStransistor serves as a floating gate. The floating gate is selectivelyprogrammed or erased to store predetermined charges. However, theelectrical erase fails to be utilized to remove the electric charges inthe floating gate. That is, for achieving the data-erasing function, theelectric charges stored in the floating gate should be removed from thefloating gate by exposing ultraviolet (UV) light to the NVM. These NVMsare named as one time programming (OTP) memories. Accordingly, the needexists for multi-times programming (MTP) memories design.

SUMMARY OF THE INVENTION

It is therefore one of the objectives of the present invention toprovide a non-volatile memory (NVM) cell structure, a NVM arraystructure and a method of fabricating a NVM cell structure to solve thedrawbacks encountered in the prior art.

According to an embodiment, a NVM cell structure is provided. The NVMcell structure includes a semiconductor substrate, a first well region,a floating gate transistor and an erase gate region. The semiconductorsubstrate has a first conductivity type, and the substrate has a firstoxide define (OD) region and a second OD region separated from eachother. The first well region is disposed in the first OD region of thesemiconductor substrate, wherein the first well region has a secondconductivity type. The floating gate transistor is disposed on the firstOD region, wherein the floating gate transistor includes a floating gateand a floating gate dielectric layer disposed between the floating gateand the first OD region, and the floating gate includes a first partoverlapping the first OD region and a second part overlapping the secondOD region. The erase gate region is disposed on the second OD region,wherein the erase gate region includes a first doped region disposed inthe second OD region and covering the second OD region, at least onesecond doped region disposed in the first doped region, and an erasegate dielectric layer between the first doped region and the floatinggate, and the first doped region encompasses the second doped region,wherein the first doped region and the second doped regions have thesecond conductivity type, and a doping concentration of each seconddoped region is larger than a doping concentration of the first dopedregion.

According to another embodiment, a NVM array structure is provided. TheNVM array structure includes a semiconductor substrate, two first wellregions, at least two floating gate transistors and an erase gateregion. The semiconductor substrate has a first conductivity type, andthe semiconductor substrate has at least two first OD region and asecond OD region separated from one another, wherein the second ODregion extends along a direction, and the at least two first OD regionsare located at two sides of the second OD region respectively. The firstwell regions are disposed in the at least two first OD regions of thesemiconductor substrate respectively, wherein each first well region hasa second conductivity type complementary to the first conductivity type.The floating gate transistors are disposed on the at least two first ODregions respectively, wherein each first floating gate transistorincludes a floating gate, and each floating gate overlapping each firstOD region and the second OD region. The erase gate region is disposed onthe second OD region, wherein the erase gate region includes at leastthree second doped regions arranged along the direction, each seconddoped region and each floating gate are sequentially arranged along thedirection alternately, and each second doped region has the secondconductivity type.

According to still another embodiment, a method of fabricating anon-volatile memory cell structure is provided. First, a substratestructure including a semiconductor substrate having a firstconductivity type, an isolation structure disposed in the semiconductorsubstrate, and a first well region having a second conductivity type isprovided, wherein the semiconductor substrate has a first OD region anda second OD region separated from each other. Next, a first doped regionis formed in the second OD region of the semiconductor substrate, andthe first doped region covers the second OD region, wherein the firstdoped region has the second conductivity type. Then, a dielectric layeris formed to cover the first OD region and the second OD region andfollowed by forming a floating gate on the dielectric layer.Subsequently, at least one second doped regions are formed in the firstdoped region, wherein the first doped region encompasses the seconddoped region, the second doped region has the second conductivity type,and a doping concentration of the second doped region is larger than adoping concentration of the first doped region.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-7 are schematic diagrams illustrating a method of fabricating anon-volatile memory (NVM) cell structure according to a first embodimentof the present invention.

FIG. 8 is a schematic diagram illustrating an equivalent circuit of theNVM cell structure according to the present invention.

FIG. 9 is a schematic diagram illustrating a plan view of the NVM arraystructure according to the first embodiment of the present invention.

FIG. 10 is across-sectional view taken along a cross-sectional line ofFIG. 9.

FIG. 11 is a schematic diagram illustrating a plan view of the NVM arraystructure according to a variant embodiment of the first embodiment ofthe present invention.

FIG. 12 illustrates a cross-sectional view taken along a cross-sectionalline E-E′ of FIG. 11.

FIGS. 13 and 14 are schematic diagrams illustrating a method offabricating a NVM cell structure according to a second embodiment of thepresent invention.

FIG. 15 is a schematic diagram illustrating a method of fabricating aNVM cell structure according to a third embodiment of the presentinvention.

FIG. 16 is a schematic diagram illustrating a method of fabricating aNVM cell structure according to a fourth embodiment of the presentinvention.

FIG. 17 is a schematic diagram illustrating a NVM cell structureaccording to a fifth embodiment of the present invention.

FIG. 18 is a schematic diagram illustrating a NVM cell structureaccording to a sixth embodiment of the present invention.

FIGS. 19 and 20 are schematic diagrams illustrating a method offabricating a NVM cell structure according to a seventh embodiment ofthe present invention.

FIGS. 21 and 22 are schematic diagrams illustrating a method offabricating a NVM cell structure according to an eighth embodiment ofthe present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are given toprovide a thorough understanding of the invention. It will, however, beapparent to one skilled in the art that the invention may be practicedwithout these specific details. Furthermore, some well-known systemconfigurations and process steps are not disclosed in detail, as theseshould be well-known to those skilled in the art. Other embodiments maybe utilized and structural, logical, and electrical changes may be madewithout departing from the scope of the present invention.

Likewise, the drawings showing embodiments of the apparatus aresemi-diagrammatic and not to scale and some dimensions are exaggeratedin the figures for clarity of presentation. Also, where multipleembodiments are disclosed and described as having some features incommon, like or similar features will usually be described with likereference numerals for ease of illustration and description thereof.

For the purposes of the present invention, The term “oxide define (OD)region” (“OD” region is sometimes referred to as “oxide defined” regionor “oxide definition” region) is commonly known in this technical fieldto be defined as a region on a silicon main surface of a substrate otherthan a local oxidation of silicon (LOCOS) or a shallow trench isolation(STI) region. The term “oxide define (OD) region” is also commonlyreferred to as an “active area” where the active circuit elements suchas transistors are formed and operated.

FIGS. 1-7 are schematic diagrams illustrating a method of fabricating anon-volatile memory (NVM) cell structure according to a first embodimentof the present invention, in which FIG. 5 is a schematic diagramillustrating a plan view of the NVM cell structure in accordance withthe first embodiment of the present invention, FIG. 6 schematicallyillustrates a cross-sectional view taken along a cross-sectional lineA-A′ of FIG. 5, and FIG. 7 schematically illustrates a cross-sectionalview taken along a cross-sectional line B-B′ of FIG. 1. As shown in FIG.1, a substrate structure 102 including a semiconductor substrate 104, afirst well region 106, and an isolation structure 108 is first provided.The semiconductor substrate 104 having a first conductivity type has afirst oxide define (OD) region 104 a and a second OD region 104 b.Specifically, the isolation structure 108 defining the first OD region104 a and the second OD region 104 b. For example, the isolationstructure 108 may be shallow trench isolation (STI) or field oxide(FOX). The first well region 106 having a second conductivity typecomplementary to the first conductivity type is disposed in the first ODregion 104 a of the semiconductor substrate 104. In this embodiment, thefirst well region 106 may be formed before the formation of theisolation structure 108, but the present invention is not limitedthereto. In another embodiment, the isolation structure 108 may beformed before the formation of the first well region 106. Also, thefirst conductivity type and the second conductivity type of thisembodiment may be p-type and n-type respectively, but not limitedherein. In another embodiment, the first conductivity type and thesecond conductivity type may be n-type and p-type respectively.

In this embodiment, an intermediate well region 110 is selectivelyformed in the semiconductor substrate 104 under the isolation structure108 and between the first OD region 104 a and the second OD region 104 bafter the isolation structure 108 is provided. Specifically, asacrificial layer 112, such as an oxide layer, is formed on thesemiconductor substrate 104, and then, a patterned photoresist layer 114may be formed on the semiconductor substrate 104. After that, an ionimplantation process of the first conductivity type using the patternedphotoresist layer 114 as a mask and a thermal drive-in process areperformed to form the intermediate well region 110. The intermediatewell region 110 may be in contact with the first well region 106, andthe bottom of the intermediate well region 110 and the bottom of thefirst well region 106 may be disposed in the same level or in differentlevels. A doping concentration of the intermediate well region 110 islarger than a doping concentration of the semiconductor substrate 104.Furthermore, the semiconductor substrate 104 may have a core deviceregion used for forming core devices, such as internal circuit, and aninput/output (I/O) device region used for forming I/O devices, and theNVM cell structure 100 of this embodiment is formed in the I/O deviceregion, but not limited thereto. Accordingly, the first well region 106may be formed with any one well region of the second conductivity typein the core device region and the I/O device region, and theintermediate well region 110 may be formed with any one well region ofthe first conductivity type in the core device region and the I/O deviceregion, but not limited herein.

As shown in FIG. 2, a first doped region 116 having the secondconductivity type is formed in the second OD region 104 b of thesemiconductor substrate 104 after the formation of the intermediate wellregion 110, and the first doped region 116 covers the second OD region104 b. Namely, the patterned photoresist layer 114 is removed after theformation of the intermediate well region 110 and followed by forminganother patterned photoresist layer 118 on the sacrificial layer 112 byusing an extra photomask. The patterned photoresist layer 118 has anopening corresponding to the second OD region 104 b. Then, another ionimplantation process of the second conductivity type using the patternedphotoresist layer 118 as a mask and another thermal drive-in process areperformed to form the first doped region 116 in the second OD region 104b.

After the first doped region 116 is formed, a second well region 120having the first conductivity type is selectively formed in thesemiconductor substrate 104 under the first doped region 116.Specifically, the second well region 120 may be formed by performinganother ion implantation process of the first conductivity type usingthe same patterned photoresist layer 118 as a mask and another thermaldrive-in process. In this embodiment, a top of the second well region120 is in direct contact with the first doped region 116, and the dopingconcentration of the second well region 120 is larger than the dopingconcentration of the semiconductor substrate 104, thereby forming a PNjunction to increase junction breakdown voltage of the NVM cellstructure 100 in the erasing operation. Preferably, a side of the secondwell region 120 is in direct contact with the intermediate well region110, and the doping concentration of the intermediate well region 110 islarger than the doping concentration of the second well region 120.

As shown in FIG. 3, the patterned photoresist layer 118 and thesacrificial layer 112 are removed sequentially, and followed by forminga dielectric layer 122 on the semiconductor substrate 104. In thisembodiment, the dielectric layer 122 may include a floating gatedielectric layer FGD on the first OD region 104 a and an erase gatedielectric layer EGD on the second OD region 104 b, but the presentinvention is not limited thereto. For example, the dielectric layer 122may be formed by a chemical vapor deposition (CVD) process and to coverthe semiconductor substrate 104 and the isolation structure 108, and amaterial of the dielectric layer 122 includes, but not limited to,silicon oxide. In another embodiment, the dielectric layer 122 may beformed by performing a thermal oxidation process, such that the first ODregion 104 a and the second OD region 104 b of the semiconductorsubstrate 104 are oxidized to form the floating gate dielectric layerFGD on the first OD region 104 a and the erase gate dielectric layer EGDon the second OD region 104 b, but the present invention is not limitedthereto. Since the floating gate dielectric layer FGD and the erase gatedielectric layer EGD are formed simultaneously, the thickness of thefloating gate dielectric layer FGD and the thickness of the erase gatedielectric layer EGD may be the same.

Furthermore, the dielectric layer 122 may be formed of a singledielectric layer or a stack of two dielectric layers, such as an I/Odielectric layer 122 a and a core dielectric layer 122 b. For example,when the dielectric layer 122 is formed of a stack of two dielectriclayers, the I/O dielectric layer 122 a is formed to cover the I/O deviceregion including the first OD region 104 a and the second OD region 104b and the core device region and followed by removing the I/O dielectriclayer 122 a in the core device region. The core dielectric layer 122 bis then formed on the I/O dielectric layer 122 a to cover the I/O deviceregion and the core device region, thereby forming the dielectric layer122 in the I/O device region.

As shown in FIG. 4, subsequently, a gate structure 124 is formed on thedielectric layer 122. The gate structure 124 may include a poly gatethat may be a single layer of polysilicon or doped polysilicon, and aspacer surrounding the poly gate. In this embodiment, the step offorming the poly gate includes forming a floating gate FG on the firstOD region 104 a and the second OD region 104 b and a select gate SG onthe first OD region 104 a. Specifically, the floating gate FG overlapsboth the floating gate dielectric layer FGD and the erase gatedielectric layer EGD, and the select gate SG is disposed on a selectgate dielectric layer SGD which is a part of the dielectric layer 122.

Since FIG. 4 cannot show the following steps, please refer to FIGS. 5-7.After the poly gate 124 is formed, another ion implantation process ofthe second conductivity type using the floating gate FG as a part of themask and another drive-in process may be performed to form at least onesecond doped region 126 having the second conductivity type in the firstdoped region 116, and another ion implantation process of the firstconductivity type using the floating gate FG and the select gate SG as apart of the mask and another drive-in process may be performed to form athird doped region 128 having the first conductivity type, a fourthdoped region 129 having the first conductivity type and a fifth dopedregion 130 having the first conductivity type in the first well region106. An erase gate region EG including the first doped region 116, thesecond doped region 126, the second well region 120 and the erase gatedielectric layer EGD, a floating gate transistor FGT including thefloating gate FG, the floating gate dielectric layer FGD, the thirddoped region 128 and the fourth doped region 129, and a selecttransistor ST including the select gate SG, the select gate dielectriclayer SGD, the fourth doped region 129 and the fifth doped region 130are accordingly formed, thereby forming the NVM cell structure 100 ofthis embodiment that is a single-poly NVM cell structure. In anotherembodiment, the sequence of the ion implantation processes of the firstconductivity type and the second conductivity type may be exchanged. Itis worthy noted that since the steps of forming the first well region106, the intermediate well region 110, the second doped region 126, thethird doped region 128, the fourth doped region 129 and the fifth dopedregion 130 can be compatible with the CMOS process, the method offabricating the NVM cell structure 100 only require the extra photomaskand two ion implantation processes to form the first doped region 116and the second well region 120. Thus, the method of this embodiment iseasy to compatible with the traditional semiconductor process and can bewidely used in different applications.

Referring to FIGS. 4-7, the NVM cell structure 100 provided by thisembodiment is further detailed in the following description, whereinFIG. 4 shows a cross-sectional view taken along a cross-sectional lineC-C′ of FIG. 5. The NVM cell structure 100 includes the semiconductorsubstrate 104 having the first OD region 104 a and the second OD region104 b thereon, the first well region 106, the floating gate transistorFGT on the first OD region 104 a and the erase gate region EG on thesecond OD region 104 b. In this embodiment, the first OD region 104 aand the second OD region 104 b are defined and surrounded by theisolation structure 108. Further, the second OD region 104 b extendsalong a first direction D1, and the first OD region 104 a extends alonga second direction D2 different from the first direction D1. Forexample, the first direction D1 and the second direction D2 aresubstantially perpendicular to each other, but not limited thereto.Also, the first well region 106 covers the first OD region 104 a andextends to be under a part of the isolation structure 108.

In the floating gate transistor FGT, the floating gate dielectric layerFGD is disposed between the floating gate FG and the first well region106, the third doped region 128 and the fourth doped region 129 aredisposed in the first well region 106 at two sides of the floating gateFG. Accordingly, a part of the first well region 106 disposed betweenthe third doped region 128 and the fourth doped region 129 serves as achannel region of the floating gate transistor FGT. The third dopedregion 128 is electrically connected to a bit line BL through a contactplug C1 and applied with a bit line voltage V_(BL). Furthermore, thefloating gate FG of the floating gate transistor FGT extends from thefirst OD region 104 a onto the second OD region 104 b such that thefloating gate FG includes a first part FG1 overlapping the first ODregion 104 a and a second part FG2 overlapping the second OD region 104b, and the floating gate FG overlaps the erase region EG. Specifically,the erase gate region EG includes two second doped regions 126 in thesecond OD region 104 b at two sides of the second part FG2 of thefloating gate FG, and the second doped regions 126, the erase gatedielectric layer EGD and the floating gate FG form an erase capacitorthat is also called metal-oxide-semiconductor (MOS) field effecttransistor (FET) capacitor. The extension direction of the first partFG1 of the floating gate FG is substantially perpendicular to theextension direction of the second part FG2 of the floating gate FG, andthe first part FG1 and the second part FG2 forms the L-shaped floatinggate FG. In another embodiment, the extension direction of the firstpart FG1 may be substantially parallel to the extension direction of thesecond part FG2. Besides, the first part FG1 of the floating gate FG ofthis embodiment crosses the first doped region 116, and thus, the seconddoped regions 126 are spaced apart from each other because the seconddoped regions 126 are formed by using the floating gate FG as the mask.Also, an overlapping area between the floating gate FG and the first ODregion 104 a may be larger than an overlapping area between the floatinggate FG and the second OD region 104 b.

In erase gate region EG, one of the second doped regions 126 iselectrically connected to an erase line EL through a contact plug C2 andapplied with an erase line voltage V_(EL) through the erase line EL.Since the first doped region 116 is disposed between the two separatedsecond doped regions 126 and connects them, the second doped regions 126are easy to be conducted to have the erase line voltage V_(EL),especially the other one of the second doped regions 126 that is notdirectly connected to the erase line EL can have the erase line voltageV_(EL). Accordingly, the electric charges Q in the floating gate FG canbe easily erased, and the erase line voltage V_(EL) can be decreased inthe erasing operation. The first doped region 116 is further disposedbetween the second doped regions 126 and the second well region 120 andseparates them, so that a PN junction is formed. Also, since a dopingconcentration of each second doped region 126 is larger than a dopingconcentration of the first doped region 116, and a doping concentrationof the second well region 120 contacting the first doped region 116 islarger than a doping concentration of the semiconductor substrate 104,the PN junction formed of the first doped region 116 and the second wellregion 120 can be stronger than the PN junction formed of the seconddoped region 126 and the semiconductor substrate 104. The stronger PNjunction may be formed between the erase capacitor and the first wellregion 106 coupled to the channel region of the floating gate transistorFGT, and the junction breakdown voltage between the second doped regions126 and the first well region 106 can be increased while applying alarge voltage difference between the second doped regions 126 and thefirst well region 106 in the erasing operation, thereby increasingcycling times of the programming operations and the erasing operationsand a lifetime of the NVM cell structure 100. The vertical junctionbreakdown the second doped regions 126 and the first well region 106 isnot easily generated. Further, since a doping concentration of theintermediate well region 110 is larger than a doping concentration ofthe second well region 120, a PN junction formed between the first wellregion 106 and the intermediate well region 110 can be used to withstandthe program voltage and avoid horizontal junction breakdown between thefirst well region 106 and the erase gate region EG during theprogramming operation.

In this embodiment, the NVM cell structure 100 further includes theselect transistor ST disposed on the first OD region 104 a. The selecttransistor ST is electrically connected to the floating gate transistorFGT through the fourth doped region 129 adjacent to the select gate SG.Specifically, the select transistor ST and the floating gate transistorFGT share the fourth doped region 129 adjacent to the select gate SG,and the fourth doped region 129 and the fifth doped region 130 aredisposed at two sides of the select gate SG, such that the fourth dopedregion 129 serves as the drain of the select transistor ST, and thefifth doped region 130 serves as the source of the select transistor ST.The fifth doped region 130 is electrically connected to a source line SLthrough at least one contact plug C3 and applied with a source linevoltage V_(SL), and the source line SL is electrically connected to thefirst well region 106. The select gate SG is electrically connected to aword line WL and applied with a word line voltage V_(WL).

Referring to FIG. 8 and TABLE 1 with FIGS. 5-7, FIG. 8 is a schematicdiagram illustrating an equivalent circuit of the NVM cell structureaccording to the present invention, and TABLE 1 lists exemplaryconductions for a programming operation, a reading operation, and anerasing operation. When the NVM cell structure 100 is operated in aprogramming operation, the bit line BL and the intermediate well region110 are grounded such that the bit line voltage V_(BL) and anintermediate well voltage V_(IM) applying to the intermediate wellregion 110 are 0 volt (V); the source line voltage V_(SL) and a firstwell voltage V₁ applying to the first well region 106 are programvoltage Vp larger than 0 V; the word line voltage V_(WL) is rangedbetween 0 to 3Vp/4, preferably Vp/2; and the erase line voltage V_(EL)is ranged between 0 to the program voltage Vp. Accordingly, the selecttransistor ST is turned on, and the program voltage is applied to thefifth doped region 130. Through channel hot electron (CHE) injectionmechanism, the electric charges Q can be injected into the floating gateFG to perform the programming operation. For example, the programvoltage Vp is ranged from 5.5 V to 9 V, preferably 7 V. When the NVMcell structure 100 is operated in an erasing operation, the bit line BL,the source line SL, the word line WL, the first well region 106 and theintermediate well region 110 are grounded, such that the bit linevoltage V_(BL), the source line voltage V_(SL), the word line voltageV_(WL), the first well voltage \h and the intermediate well voltageV_(IM) are 0 V, and the erase line voltage V_(EL) is an erase voltage Veranged from 9 V to 18 V. Through the voltage difference between thesecond doped regions 126 and the first well region 106, the electriccharges Q in the floating gate FG can tunnel into the second dopedregions 126 with Fowler-Nordheim (FN) tunneling mechanism, therebyerasing the electric charges Q in the floating gate FG. When the NVMcell structure 100 is operated in a reading operation, the source linevoltage V_(BL) and the first well voltage V₁ are read voltage Vr rangedfrom 1.2 V to 3.3 V, preferably 2.0 V or 2.6 V and the bit line BL, theword line WL, the erase line EL and the intermediate well region 110 aregrounded, such that the bit line voltage V_(BL), the word line voltageV_(WL), the erase line voltage V_(EL) and the intermediate well voltageV_(IM) are 0 V. According to the above operation modes, the electriccharges Q can be electrically programmed to the floating gate andelectrically erased from the floating gate. Accordingly, the NVM cellstructure of this embodiment can be may be a multi-time programmable(MTP) memory.

TABLE 1 V_(BL) V_(SL) V_(WL) V₁ V_(EL) V_(IM) programming 0 Vp 0~3 Vp/4VP 0~Vp 0 operation erasing 0 0 0 0 Ve 0 operation reading 0 Vr 0 Vr 0 0operation

According to the above-mentioned NVM cell structure 100, a NVM arraystructure is also provided in the present invention. Referring to FIGS.9 and 10, FIG. 9 is a schematic diagram illustrating a plan view of theNVM array structure according to the first embodiment of the presentinvention, and FIG. 10 is a cross-sectional view taken along across-sectional line D-D′ of FIG. 9. As shown in FIGS. 9 and 10, the NVMarray structure 200 includes at least two NVM cell structures 100 of thefirst embodiment on the semiconductor substrate 104. Also, the second ODregions 104 b of the NVM cell structures 100 are connected in series andform a single second OD region 104 b, and the erase gate regions EG ofthe NVM cell structures 100 are connected in series and form a singleerase gate region EG. Thus, the first doped regions 116 and the secondwell regions 126 are also connected to form single first doped region116 and the single second well region 126 respectively. In order toachieve the performance of the first embodiment, the first doped region116 cover the second OD region 104 b, and the second well region 126 isdisposed under the first doped region 116. Besides the second OD region104 b, the semiconductor substrate 104 further has at least two first ODregions 104 a separated from one another, wherein the second OD region104 b extends along a first direction D1, and the first OD regions 104 aare located at a first side and a second side of the second OD region104 b opposite to each other respectively. For instance, the first ODregions 104 a can be symmetrical to each other with respect to thesecond OD region 104 b. The NVM array structure 200 may further includetwo first well regions 106 disposed in the first OD regions 104 a of thesemiconductor substrate 104 respectively. The first well regions 106 maybe symmetrical to each other with respect to the second OD region 104 b.Each floating gate transistor FGT in each NVM cell structure 100 isrespectively disposed on each first OD region 104 a. Each floating gatetransistor FGT includes a floating gate FG overlapping each first ODregion 104 a and extending to overlap the second OD region 104 b. Also,the erase gate region EG includes at least three second doped regions126 arranged along the first direction D1, and each second doped region126 and each floating gate FG are sequentially arranged along the firstdirection D1 alternately.

In this embodiment, the NVM array structure 200 may include a pluralityof NVM cell structures 100 arranged in an array formation. Two rows ofthe NVM cell structures are taken as an example in the followingdescription, but not limited thereto. Specifically, the semiconductorsubstrate 104 has a plurality of the first OD regions 104 a separatedfrom one another and disposed at the first and second sides of thesecond OD region 104 b respectively. Each floating gate transistor FGTis disposed on each first OD region 104 a respectively, and each selecttransistor ST is disposed on each first OD region 104 a respectively.The erase gate region EG includes a plurality of second doped regions126. Also, each floating gate FG crosses the second OD region 104 b, sothat one of the second parts FG2 of the floating gates FG of any two ofthe erase capacitors disposed at two sides of the second OD region 104 band corresponding to each other has a zigzag part overlapping the secondOD region 104 b as shown in FIG. 9.

For example, four NVM cell structures 100 may form a repeated unit 200 aand may be divided into first, second, third and fourth NVM cellstructures 100 a, 100 b, 100 c, 100 d. The first and second NVM cellstructures 100 a, 100 b are arranged along the second direction D2, andthe third and fourth NVM cell structures 100 c, 100 d are also arrangedalong the second direction D2. In order to prevent the floating gates FGof the first and second NVM cell structures 100 a, 100 b or the thirdand fourth NVM cell structures 100 c, 100 d from contacting each other,the second part FG2 of the floating gate FG in the first NVM cellstructure 100 a and the second part FG2 of the floating gate FG in thefourth NVM cell structure 100 d have a zigzag shape such that the secondparts FG2 of the first and fourth NVM cell structures 100 a, 100 d aresymmetrical to each other with respect to a center of the repeated unit200 a, but the present invention is not limited thereto. In anotherembodiment, the zigzag parts of the floating gates FG may be in thesecond and third NVM cell structures 100 b, 100 c, the first and thirdNVM cell structures 100 a, 100 c or the second and fourth NVM cellstructures 100 b, 100 d.

In addition, the select gates SG of the select transistors ST can beconnected in series and form a word line WL extending along the firstdirection D1. Also, at least one contact plug C3 is disposed on eachfifth doped region 130 in each first OD region 104 a, so that the fifthdoped regions 130 arranged along the first direction D1 are electricallyconnected to the same source line SL. The third doped regions 128farther away from the word line WL and arranged along the seconddirection D2 are electrically connected to the same bit line BL throughcontact plugs C1. The second doped regions 126 at the centers of therepeated units 200 a respectively are electrically connected to the sameerase line EL through contact plugs C2.

Referring to FIGS. 11 and 12, FIG. 11 is a schematic diagramillustrating a plan view of the NVM array structure according to avariant embodiment of the first embodiment of the present invention, andFIG. 12 illustrates a cross-sectional view taken along a cross-sectionalline E-E′ of FIG. 11. As shown in FIGS. 11 and 12, as compared with theabove-mentioned first embodiment, the second part FG2 of each floatinggate FG in this variant embodiment may not cross the second OD region104 b and the first doped region 116. Specifically, an edge of eachfloating gate FG is disposed right on the second OD region 104 b.Therefore the second doped region 126 may be formed by implanting by theaid of floating gate FG which is as an implant mask. Since each floatinggate FG doesn't cross the second OD region 104 b, the overlapping areabetween each floating gate FG and the first doped region 116 is reduced,and the coupling capacitance between each floating gate FG and the firstdoped region 116 is accordingly decreased as compared with the firstembodiment. By doing so, the voltage of the floating gate FG coupled tothe first well region 106 is larger, thereby increasing the differencebetween the erase line voltage V_(EL) and the voltage of the floatinggate FG during the erasing operation. Thus, the erase of the electriccharges in the floating gate FG is easier, and the erase line voltageV_(EL) may be reduced to increase the lifetime of the NVM arraystructure 200′.

The NVM cell structure and the method of fabricating the same of thepresent invention are not limited to the above-mentioned embodiment. Thefollowing description continues to detail the other embodiments orvariant embodiments, and in order to simplify and show the differencebetween the other embodiments or variant embodiments and theabove-mentioned embodiment, the same numerals denote the same componentsin the following description, and the same parts are not detailedredundantly.

Referring to FIGS. 13 and 14, FIGS. 13 and 14 are schematic diagramsillustrating a method of fabricating a NVM cell structure according to asecond embodiment of the present invention, where FIG. 14 illustrates atop view of the NVM cell structure according to the second embodiment ofthe present invention. As shown in FIG. 13, the step of forming secondwell region 120 and the steps before forming the second well region 120in the method of fabricating the NVM cell structure 300 of thisembodiment are the same as the method in the first embodiment, and arenot detailed redundantly. As compared with the first embodiment, thestep of forming the second doped regions 126 may be performed betweenforming the first doped region 116 and removing the patternedphotoresist layer 118 and the sacrificial layer 112 in the method offabricating the NVM cell structure of this embodiment. In thisembodiment, after the second well region 120 is formed, an ionimplantation process of the second conductivity type and a thermaldrive-in process are performed through using the same patternedphotoresist layer 118 as a mask to form the second doped region 126.Then, as shown in FIG. 14, the step of removing the patternedphotoresist layer 118 and the sacrificial layer 112 and the stepsthereafter in the method of fabricating the NVM cell structure 300 ofthis embodiment are the same as the method in the first embodiment andare not detailed redundantly.

Referring to FIG. 15 together with FIGS. 5-7, FIG. 15 is a schematicdiagram illustrating a method of fabricating a NVM cell structureaccording to a third embodiment of the present invention. As shown inFIG. 15, the step of forming the intermediate well region 110 and thesteps before forming the intermediate well region 110 in the method offabricating the NVM cell structure of this embodiment are the same asthe method in the first embodiment and are not detailed redundantly. Ascompared with the first embodiment, the step of removing the patternedphotoresist layer 118 and the sacrificial layer 112 is performed next tothe step of forming the intermediate well region 110 and followed byforming the I/O dielectric layer 122 a on the first OD region 104 a andthe second OD region 104 b in this embodiment. After that, anotherpatterned photoresist layer 418 is formed on the I/O dielectric layer122 a, and then, by using the patterned photoresist layer 418 as a mask,the first doped region 116 is formed in the second OD region 104 b andcovers the second OD region 104 b. Next, the second well region 120 isformed in the semiconductor substrate 104 under the first doped region116. As shown in FIG. 5-7, after the second well region 120 is formed,the core dielectric layer 122 b is formed on the I/O dielectric layer122 a, thereby forming the dielectric layer 122. Later, the poly gateincluding the floating gate FG is formed on the dielectric layer 122 andfollowed by forming the second doped regions 126 in the first dopedregion 116. The steps of forming the poly gate and forming the seconddoped regions 126 in this embodiment are the same as the firstembodiment, and are not detailed redundantly.

Referring to FIG. 16 together with FIG. 14, FIG. 16 is a schematicdiagram illustrating a method of fabricating a NVM cell structureaccording to a fourth embodiment of the present invention. As shown inFIG. 16, the step of forming the second well region 120 and the stepsbefore forming the second well region 120 in the method of fabricatingthe NVM cell structure of this embodiment are the same as the method inthe third embodiment and are not detailed redundantly. As compared withthe third embodiment, the step of forming the second doped regions 126may be performed next to the step of forming the first doped region 116in the method of fabricating the NVM cell structure of this embodiment.In this embodiment, after the second well region 120 is formed, an ionimplantation process of the second conductivity type and a thermaldrive-in process are performed through using the same patternedphotoresist layer 418 as a mask to form the second doped region 126.Then, the step of forming the core dielectric layer 122 b and the stepsafter forming the core dielectric layer 122 b are the same as the thirdembodiment and are not detailed redundantly, thereby forming the NVMcell structure that is the same as the NVM cell structure 300 of thesecond embodiment as shown in FIG. 14.

Referring to FIG. 17, FIG. 17 is a schematic diagram illustrating a NVMcell structure according to a fifth embodiment of the present invention.As shown in FIG. 17, in the NVM cell structure 500 provided by thisembodiment, no intermediate well region is disposed between the firstwell region 106 and the second well region 120. In the method offabricating the NVM cell structure 500 of this embodiment, the step offorming the first doped region 116 is performed next to the step ofproviding the substrate structure 102 and followed by forming the secondwell region 120. Accordingly, a side of the second well region 120 maybe in contact with the first well region 106.

Referring to FIG. 18, FIG. 18 is a schematic diagram illustrating a NVMcell structure according to a sixth embodiment of the present invention.As shown in FIG. 18, in the NVM cell structure 600 provided by thisembodiment, no second well region is formed under the first doped region116. In the method of fabricating the NVM cell structure 600 of thisembodiment, the step of removing the patterned photoresist layer 118 andthe sacrificial layer 112 and the step of forming the dielectric layer122 is performed next to the step of forming the first doped region 116.Accordingly, a bottom of the first doped region 116 and a side of theintermediate well region 110 may be in contact with the semiconductorsubstrate 104 between them. In order to avoid the horizontal junctionbreakdown between the first well region 120 and the erase gate region EGduring the programming operation, a width of the intermediate wellregion 110 or a width of the isolation structure 108 between the firstwell region 106 and the erase gate region EG in this embodiment may belarger than a width of the intermediate well region 110 and the width ofthe isolation structure 108 between the first well region 106 and theerase gate region EG in the first embodiment.

Referring FIGS. 19 and 20, FIGS. 19 and 20 are schematic diagramsillustrating a method of fabricating a NVM cell structure according to aseventh embodiment of the present invention, where FIG. 20 is aschematic diagram illustrating a cross-sectional view of the NVM cellstructure according the seventh embodiment of the present invention. Asshown in FIG. 19, as compared with the first embodiment, the step ofproviding the substrate structure 702 further includes providing a firstdeep well region 704 in the semiconductor substrate 104 under the firstwell region 106, and the first deep well region 704 has the secondconductivity type. As shown in FIG. 21, the steps after providing thesubstrate structure 702 of this embodiment are the same as the method ofthe first embodiment as shown in FIGS. 2-4 or the method of the thirdembodiment as shown in FIG. 16, and are not redundantly detailed. In NVMcell structure 700 of this embodiment, the first deep well region 704 isdisposed under the first well region 106, the intermediate well region110 and the second well region 120 and used to electrically insulate theNVM cell structure 700 from other devices formed in the samesemiconductor substrate 104.

Referring FIGS. 21 and 22, FIGS. 21 and 22 are schematic diagramsillustrating a method of fabricating a NVM cell structure according toan eighth embodiment of the present invention, where FIG. 22 is aschematic diagram illustrating a cross-sectional view of the NVM cellstructure according the eighth embodiment of the present invention. Asshown in FIG. 21, as compared with the seventh embodiment, the step ofproviding the substrate structure 802 further includes providing asecond deep well region 804 in the semiconductor substrate 104 betweenthe first well region 106 and the first deep well region 704, and thesecond deep well region 804 has the first conductivity type. In thisembodiment, a doping concentration of the second deep well region 804 islarger than the doping concentration of the semiconductor substrate 104and less than the doping concentration of the intermediate well region110. As shown in FIG. 22, the steps after providing the substratestructure 802 of this embodiment are the same as the method of the sixthembodiment, and are not redundantly detailed. In the NVM cell structure800 of this embodiment, besides between the first well region 106 andthe first deep well region 704, the second deep well region 804 is alsodisposed between the first doped region 116 and the first deep wellregion 704, such that the vertical junction breakdown between the firstdoped region 116 and the first deep well region 704 can be prevented,and the NVM cell structure 800 without the second well region 120disposed between the first doped region 116 and the first deep wellregion 704 still can tolerate the high voltage applied to the seconddoped region 126 during the erasing operation. In another embodiment,the steps after providing the substrate structure 802 may be the same asthe method of the first embodiment or the method of the thirdembodiment, so that the NVM cell structure 800 may include the secondwell region 120 between first doped region 116 and the second deep wellregion 804. The doping concentration of the second deep well region 804may be substantially the same as the doping concentration of the secondwell region 120, but not limited herein.

From the above description, in the NVM cell structure of the presentinvention, the electric charges Q can be electrically programmed to thefloating gate and electrically erased from the floating gate, so thatthe NVM cell structure can be multi-time programmable (MTP) and be anelectrically erasable programmable (EEP) NVM. Furthermore, since thefirst doped region having lower doping concentration than the seconddoped region is disposed to encompass the second doped region and thesecond well region having the doping concentration larger than that ofthe semiconductor substrate and less than that of the intermediate wellregion is disposed between the second doped regions and the first wellregion, the stronger PN junction may be formed between the erasecapacitor and the first well region coupled to the channel region of thefloating gate transistor, and the junction breakdown voltage between thesecond doped regions and the first well region can be increased whileapplying a large voltage difference between the second doped regions andthe first well region in the erasing operation, thereby increasingcycling times of the programming operations and the erasing operationsand a lifetime of the NVM cell structure.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A method of fabricating a NVM cell structure,comprising: providing a substrate structure comprising a semiconductorsubstrate having a first conductivity type, an isolation structuredisposed in the semiconductor substrate, and a first well region havinga second conductivity type, wherein the semiconductor substrate has afirst OD region and a second OD region separated from each other;forming a first doped region in the second OD region of thesemiconductor substrate, and the first doped region covering the secondOD region, wherein the first doped region has the second conductivitytype; forming a dielectric layer to cover the first OD region and thesecond OD region; forming a floating gate on the dielectric layer;forming at least one second doped region in the first doped region,wherein the first doped region encompasses the second doped region, thesecond doped region has the second conductivity type, and a dopingconcentration of the second doped region is larger than a dopingconcentration of the first doped region; and forming a second wellregion in the semiconductor substrate under the first doped region afterforming the first doped region, wherein a top of the second well regionbeing in direct contact with the first doped region, the second wellregion has the first conductivity type and a doping concentration of thesecond well region is larger than a doping concentration of thesemiconductor substrate.
 2. The method of fabricating the NVM cellstructure according to claim 1, further comprising forming anintermediate well region in the semiconductor substrate under theisolation structure and between the first well region and the secondwell region between providing the substrate structure and forming thefirst doped region, wherein the intermediate well region has the firstconductivity type, and a doping concentration of the intermediate wellregion is larger than the doping concentration of the second wellregion.
 3. The method of fabricating the NVM cell structure according toclaim 1, wherein the step of forming the first doped region and the stepof forming the second well region are performed before forming thedielectric layer.
 4. The method of fabricating the NVM cell structureaccording to claim 3, wherein the step of forming the second dopedregion is performed between forming the first doped region and formingthe dielectric layer.
 5. The method of fabricating the NVM cellstructure according to claim 1, wherein the step of forming thedielectric layer comprises forming an I/O dielectric layer on the firstOD region and the second OD region, and forming a core dielectric layeron the I/O dielectric layer, and wherein the step of forming the firstdoped region and the step of forming the second well region areperformed between the step of forming the I/O dielectric layer and thestep of forming the core dielectric layer.
 6. The method of fabricatingthe NVM cell structure according to claim 5, wherein the step of formingthe second doped region is performed between the step of forming thefirst doped region and the step of forming the core dielectric layer. 7.The method of fabricating the NVM cell structure according to claim 1,wherein providing the substrate structure further comprises providing afirst deep well region in the semiconductor substrate under the firstwell region, and the first deep well region has the second conductivitytype.
 8. The method of fabricating the NVM cell structure according toclaim 7, wherein providing the substrate structure further comprisesproviding a second deep well region in the semiconductor substratebetween the first well region and the first deep well region, the seconddeep well region has the first conductivity type, and a dopingconcentration of the second deep well region is larger than the dopingconcentration of the semiconductor substrate.
 9. The method offabricating the NVM cell structure according to claim 1, furthercomprising forming an intermediate well region in the semiconductorsubstrate between providing the substrate structure and forming thefirst doped region, wherein the intermediate well region is disposedunder the isolation structure between the first OD region and the secondOD region, the intermediate well region is in direct contact with thefirst well region, and a doping concentration of the intermediate wellregion is larger than a doping concentration of the semiconductorsubstrate.
 10. The method of fabricating the NVM cell structureaccording to claim 1, further comprising forming a third doped regionand a fourth doped region in the first well region at two sides of thefloating gate after forming the floating gate, wherein the third dopedregion and the fourth doped region both have the first conductivitytype.
 11. The method of fabricating the NVM cell structure according toclaim 10, wherein forming the floating gate further comprises forming aselect gate on a select gate dielectric layer, and forming a fifth dopedregion in the first well region at the same time of the third dopedregion and fourth doped region, wherein the select gate is disposedbetween the fourth doped region and the fifth doped region.
 12. A methodof fabricating a NVM cell structure, comprising: providing a substratestructure comprising a semiconductor substrate having a firstconductivity type, an isolation structure disposed in the semiconductorsubstrate, and a first well region having a second conductivity type,wherein the semiconductor substrate has a first OD region and a secondOD region separated from each other; forming a first doped region in thesecond OD region of the semiconductor substrate, and the first dopedregion covering the second OD region, wherein the first doped region hasthe second conductivity type; forming a dielectric layer to cover thefirst OD region and the second OD region; forming a floating gate on thedielectric layer; forming at least one second doped region in the firstdoped region, wherein the first doped region encompasses the seconddoped region, the second doped region has the second conductivity type,and a doping concentration of the second doped region is larger than adoping concentration of the first doped region; and forming anintermediate well region in the semiconductor substrate betweenproviding the substrate structure and forming the first doped region,wherein the intermediate well region is disposed under the isolationstructure between the first OD region and the second OD region, theintermediate well region is in direct contact with the first wellregion, and a doping concentration of the intermediate well region islarger than a doping concentration of the semiconductor substrate.
 13. Amethod of fabricating a NVM cell structure, comprising: providing asubstrate structure comprising a semiconductor substrate having a firstconductivity type, an isolation structure disposed in the semiconductorsubstrate, and a first well region having a second conductivity type,wherein the semiconductor substrate has a first OD region and a secondOD region separated from each other; forming a first doped region in thesecond OD region of the semiconductor substrate, and the first dopedregion covering the second OD region, wherein the first doped region hasthe second conductivity type; forming a dielectric layer to cover thefirst OD region and the second OD region; forming a floating gate on thedielectric layer; forming at least one second doped region in the firstdoped region, wherein the first doped region encompasses the seconddoped region, the second doped region has the second conductivity type,and a doping concentration of the second doped region is larger than adoping concentration of the first doped region; and forming a thirddoped region and a fourth doped region in the first well region at twosides of the floating gate after forming the floating gate, wherein thethird doped region and the fourth doped region both have the firstconductivity type; wherein forming the floating gate further comprisesforming a select gate on a select gate dielectric layer, and forming afifth doped region in the first well region at the same time of thethird doped region and fourth doped region, wherein the select gate isdisposed between the fourth doped region and the fifth doped region.